The need for nanocrystals with bright and stable fluorescence for various applications covering biology to electrooptics is increasingly growing. This is particularly true for III-V semiconductor nanocrystals that can cover the technologically important visible to near infrared (NIR) spectral ranges and at the same time be used for large scale implementation stemming from their environmentally benign (e.g. InP) character.
The main strategy to increase photoluminescence quantum yield and stability of the nanocrystals is to grow a passivating shell on the cores surface. This removes surface defects acting as traps for the carriers and therefore reduces the probability for the undesired processes of emission quenching via nonradiative decay. Moreover, the passivating shell protects the core and reduces surface degradation. Two main factors are considered while choosing the semiconductor material for the passivating shell: the first is the lattice mismatch between the core and shell materials. A large lattice mismatch will cause strain at the core/shell interface that can lead to creation of defect sites acting as trap sites for the charge carriers. The second factor is the band offsets between the core and shell regions that should be sufficiently high so that carriers are confined into the core region and kept separated from the surface where defects can lead to the undesired nonradiative relaxation processes. This latter effect is particularly critical in III-V semiconductors that typically are characterized by small effective masses for the charge carriers requiring a large potential barrier for their confinement.
Earlier work on core/shell nanocrystals resulted in quantum yield values of up to 90% for II-VI/II-VI core/shell structures and up to 20% for III-V/II-VI core/shell structures (Banin et al in WO02/25745 and by Haubold et al in Chem. Phys. Chem., (2001) 2, 331). For the III-V structures there is still significant room for improvement in the quantum yield values, but even for the II-VI structures showing high quantum yield, the shell thickness corresponding to these maximal quantum yields is small, typically only of about 2 monolayers. This limitation is likely due to traps created by structure imperfections formed in the growth process. A thick shell is important for the stability of the nanocrystals, especially for applications in which they are exposed to tough processes.
A solution to this problem was given in the work of Li et al (J. Am. Chem. Soc., (2003) 125, 12567-12575), in which a layer-by-layer growth method was used. A layer-by-layer growth was previously also used to create CdS/HgS/CdS quantum dot-quantum well structures (Mews et al., J. Phys. Chem., (1994) 98, 934). In this method the cation and anion shell precursors are added sequentially into the reaction vessel. Another solution to the problem of increased stress with shell thickness is to grow a heteroshell structure in which a buffer layer is used to decrease stress in the shell.
In international publication no. WO04/066361 high photoluminescence quantum yield compositions containing monodispersed colloidal core/shell semiconductor nanocrystals and doped or radially-doped core/multishell nanocrystals are disclosed. The preparation of multi-shelled structures such as the core/shell/shell containing CdSe cores has also been demonstrated, for example by Talapin et al in J. Phys. Chem. (2004) B 108, 18826 and by Mews et al in J. Amer. Chem. Soc., (2005) 127, 7480-7488.